Pregelatinized starches are generally prepared by thermal, chemical or mechanical techniques which can cause swelling of the starch granules so that they are soluble in cold water.
The preferred techniques are spray-drying, cooking on a drum or extrusion. Autoclaving or indirect heating on a heat exchanger are cooking processes which tend to produce complex colloidal dispersions consisting of intact, fragmented and swollen granules.
It is well known that high amylose starches, that is to say which have more than 50% by weight of amylose, are particularly difficult to disperse and require high cooking temperatures.
The document U.S. Pat. No. 3,086,890 describes a process for preparing pregelatinized amylose. This process consists in autoclaving at 191° C., under pressure, a solution of amylose containing at most 25% by weight of dry matter, and then drum-drying at 110–200° C. The powders obtained are amorphous and form irreversible gels after dispersion. Their apparent density is high, that is to say generally greater than 0.4 g/ml.
The document U.S. Pat. No. 3,607,394 describes a process for the pregelatinization of starches having amylose contents of less than 60% by weight. This process consists in cooking the starch at 149° C., followed by drum-drying, spray-drying or another type of drying means.
The document EP 0 366 898 describes a coupled cooking/spray-drying process which makes it possible to obtain amorphous products, which are practically free of retrogradation, and which have an apparent density greater than that of an identical starch which has been subjected to the same process but in two separate stages, that is an apparent density greater than 0.45 g/ml. The cooking of high amylose starches is carried out on a jet cooker at 143° C. and the spray-drying is carried out with an inlet air temperature of 220° C. This document presents, in FIG. 7, a comparison of various prior art pregelatinized starches.
Starting from this teaching, the applicants have now found, after long research studies, that it was possible to prepare a pregelatinized high amylose starch using a particular process leading to products of low apparent density while working at not very high temperatures, which none of the prior art techniques made it possible to obtain.